Reveal UN terribly evil (6)
I believe the answer is:
unveil
'reveal' is the definition.
(I know that reveal can be written as unveil)
'un terribly evil' is the wordplay.
'terribly' is an anagram indicator.
'evil' anagrammed gives 'veil'.
'un'+'veil'='UNVEIL'
(Other definitions for unveil that I've seen before include "Expose by removing covering" , "Remove covering, expose" , "Introduce publicly for the first time" , "Reveal by removing a curtain" , "Remove headdress" .)